This invention relates to a double-focussing mass spectrometer with a large entrance aperture.
Known mass spectrometers with focussing of direction and energy (referred to as "double-focussing" mass spectrometers) are able to admit only those ions which are emitted by the ion source in a small solid-angle range about the axis of entry of the mass spectrometer. In many applications this limitation is unimportant, especially when the ions under analysis can be accelerated to an energy which is high in comparison with their initial energy, since the paths of the accelerated particles take up only a small solid angle.
There are cases, however, in which the ions cannot be accelerated before their mass is analysed. If the ions are emitted by the ion source in a large solid angle, known mass spectrometers will accept only a very small proportion of particles (acceptance factor; ratio of the particles analysed by the mass spectrometer to the total number of particles emitted by the ion source). Assume, for example, an ion source with a small source volume, which emits a flow of n.sub.0 ions per second into one hemisphere. Under these circumstances, in the emission distribution according to the cosine law, only the small fraction dn/n.sub.o = .alpha..sup.2 comes within the solid angle about the axis of entry of the mass spectrometer, which is given by a circular cone with the small aperture angle .alpha.. If, however, a solid angle is considered which is limited by the wall of a hollow cone with a mean vertex angle .phi. and the same aperture angle .alpha., then dn/n.sub.o = 2 sin2.phi...alpha. particles are picked up by this hollow cone. dn/n.sub.o = 2.alpha. results for .phi. = 45.degree.. This is more than in the first case by the factor 2/.alpha., e.g. by the factor 50 for .alpha.=2.3.degree..
A mass spectrometer capable of accepting the ions from a solid angle with the shape of a hollow cone would, therefore, have a considerably larger entrance aperture or "light intensity" than known mass spectrometers that are able to accept only particles from a circular cone with the aperture angle .alpha..
There are, of course, magnetic beta spectrometers ("Alpha-, Beta- and Gamma-Ray Spectroscopy," edited by Kai Siegbahn, 1965, North-Holland Publishing Company, Amsterdam, Volume 1, pages 126 to 135) and electrostatic electron-beam spectrometers (E. Blauth, Zeitung der Physik, Vol. 147 (1957), pages 228-240), in which the electrons emitted in the shell of a hollow cone are focussed on a detecting arrangement. Such electron-beam spectrometers are by their nature intended only for the energy analysis of a single type of particle, namely electrons, and are not suitable for analysing ions with various charge-to-mass ratios.
The problem of the present invention is the design of a double-focussing mass spectrometer with a large entrance aperture, hence a mass spectrometer with focussing of direction and energy, which has a large entrance aperture and, consequently, is able to absorb a high percentage of the particles which are emitted by an ion source within a large solid angle, e.g. in a hemisphere.